1. Field of the Invention
The present invention relates to plasma engineering and more particularly it comprehends an arc plasma generator and a plasma arc apparatus for treating the surfaces of work-pieces incorporating the same arc plasma generator.
The invention may prove most advantageous in surfacing, cleaning and etching metal surfaces in vacuum. Specifically, vacuum condensation of the plasma of metals is used to produce antifriction, corrosion-resistant, abrasion-resistant, heat-resistant, superconducting, optical, and other coatings.
2. Description of the Prior Art
Heretofore attempts to treat the metal surfaces having the roughness height from 0.025 to 0.5 .mu.m by plasma streams produced by the arc plasma generators have failed to retain the roughness number of the surfaces. This is due to the presence of an appreciable number of macroparticles--droplets and solid fragments of the cathode material--in the plasma stream generated by the cathode spot. Apart from worsening the surface finish of the coatings applied by the plasma technique the macroparticles give rise to punctures and build-up portions in the condensates to thereby degrade mechanical, electrical, optical and other properties of the coatings and in practice the expected advantages of applying various coatings are not gained in full (resistance to wear, superior antifriction and corrosion-resistant properties, etc.).
Known in the art is an arc plasma generator disclosed in U.S. Pat. No. 3,625,848. This generator comprises a consumable cathode arranged inside the anode in axial alignment therewith, a means for exciting an arc between the cathode and the anode (ignition means), and a power unit for maintaining the arc electrically connected to the anode and the cathode. The ignition means initiates an arc discharge between the cathode and the anode. The cathode generates a plasma comprising atoms and ions of the cathode material. As has already been stated the plasma stream contains an appreciable number of macroparticles--droplets and solid fragments of the cathode material--which are undesirable since they impair the quality of the coatings.
The macroparticles in the plasma stream are the result of a local and the general superheating of the active surface of the cathode by such a powerful and concentrated thermal source as the cathode spot is (the spot's temperature is several thousand degrees, current density is as great as from about 10.sup.6 to about 10.sup.7 A/cm.sup.2).
Known in the art is also an arc plasma generator (A. S. Gilmour, Jr., D. L. Lockwood, "Pulsed Metallic-Plasma Generators", PROCEEDINGS OF THE IEEE, Vol. 60, No. 8, August, 1972, pages 977-991) comprising a consumable cathode arranged coaxially with a cylindrical anode, an igniting electrode for initiating an arc between the cathode and the anode connected to an igniting pulse generator, and a power unit for maintaining the arc. The arc plasma generator further comprises a focusing solenoid arranged on the anode.
The arc plasma generator is put into operation by applying a firing pulse to the igniting electrode with a frequency of the order of a few tens of Hertz. The plasma stream is generated at the same frequency as soon as a pulsed arc discharge between the anode and the cathode is excited. The macroparticles in the metallic plasma present a problem with this arc plasma generator. The direction of the velocity vector of the charged components of the stream can be varied by turning the axis of the focusing solenoid through an angle with respect to the axis of the system. In this case a number of macroparticles are removed from the plasma stream as a result of a certain separation of the streams of macroparticles and charged components (ions and electrons) in space. However, the prior art generator does not exclude all the macroparticles from reaching the generator outlet and, consequently, from hitting a substrate since the range of turning the focusing solenoid (15.degree. and up) makes the system transparent from the cathode to the generator outlet for the macroparticles rushing in the same direction.